The present invention relates to variable displacement hydrostatic pumps and displacement varying controls therefor, and more particularly, to such pumps which are of the “tandem” pump type.
Although the present invention may be utilized with variable displacement hydrostatic pumps of different types (i.e., wherein the actual pumping element or “rotating group” can be any one of several different types), the present invention is especially adapted for use with axial piston pumps of the “swashplate” type (or the swash and cradle type), and will be described in connection therewith.
A typical variable displacement axial piston pump, especially of the type used in mobile applications, includes some sort of operator-controlled input device, such as a joy stick or manually operable handle. By means of such an input, the operator can change the tilt angle of the swashplate, thus varying the displacement of the pump (i.e., the volume of fluid pumped per revolution of the input shaft). Typically, the actual movement of the swashplate on such a pump is accomplished by a fluid pressure responsive device, such as a servo-piston, or a pair of servo-cylinders. In either case, when the operator moves the input control, the result is that control pressure (normally the outlet pressure from the charge pump) is communicated to the fluid pressure operated device to either increase or decrease the tilt angle of the swashplate, depending upon the direction of movement of the input control.
Within the mobile hydraulics industry, there is a certain range of sizes (pump displacements) for axial piston pumps which has become the standard within the industry. For example, there are the relatively smaller axial piston pumps having displacements in the range of about 2.0 to about 3.0 cubic inches per revolution, and there are the relatively larger axial piston pumps having displacements in the range of about 7.0 to about 9.0 cubic inches per revolution. Therefore, each manufacturer of axial piston pumps provides a pump model corresponding to the relatively smaller displacement noted above, a pump model corresponding to the relatively larger displacement noted above, and a number of models in between the two displacement extremes, as described above. The various axial piston pump models offered by each manufacturer satisfy the vast majority of the mobile application requirements for such pumps. However, there are occasionally vehicle applications which require a pump having a substantially larger displacement than the largest size normally available for mobile hydraulic uses.
In order to meet the occasional, relatively lower volume applications which require much larger pump displacements, those skilled in the art have developed “tandem” pumps in which a single pump assembly houses two separate (but typically identical) axial piston rotating groups, both driven by a single, common input shaft (which also drives the charge pump, as is well known to those skilled in the art).
Although such tandem pumps can be plumbed in several different ways, and the present invention could be utilized in connection with practically any type of tandem pump, it is especially suited for use in connection with a tandem pump in which the output flows of the two rotating groups are combined, and communicated to a single load (e.g., a hydraulic motor), as opposed to the situation in which the output flow of each rotating group is utilized by a different load. Therefore, the present invention will be described in connection with, and is preferably used with, a tandem pump having both rotating groups communicated to a common pump outlet port.
It has been common practice in the tandem pumps sold commercially, prior to the present invention, to include with each of the pumping elements (the “rotating groups”) all of the various pump control features required for operation of the pump. For example, if the tandem pump requires, as is normally the case, a high pressure relief valve and check valve assembly, it has been common practice to include such an assembly in the end cap of each of the axial piston rotating groups, such that the overall tandem pump assembly actually is provided with two sets of high pressure relief valves and check valves. Among the pump controls commonly required for tandem pumps is some sort of internal pressure override (IPOR) control, by means of which an excessive load pressure will result in a reduction in the pump displacement (i.e., a reduction in the tilt angle of the swashplate).
Although there is generally no functional disadvantage in providing a tandem pump assembly with redundant pump controls, such redundancy does add substantially to the overall cost of the tandem pump and in most cases may also be-undesirable in terms of packaging, weight, and the overall size of the tandem pump assembly.